Sorry to correctly you David but some of that is wrong -

Each Cell of a LiPo (not Sack), has a nominal voltage of 3.7V (This is the voltage that anything plugged into the LiPo will see when its under load). When fully charged a LiPo has a voltage of ~4.2V per Cell. You never really want a cells voltage to drop under that 3.7V nominal, but its not the end of the world if it does go to say 3.5V or even 3.4V, anything lower and you can start to cause damage. Some ESC's have built in cut offs that you can adjust to your liking, the default is usually 3.2V per cell.

LiPo's must be balanced when you have more than 1 Cell wired together, in series or parallel (Which in the case of fighting robots is 99% of the time unless you are talking about Antweights). Cells at different voltages have different internal resistances and are also trying to move electrons at different speeds, which will cause damage, produce heat, and potentially cause fires.

The balancing process is done automatically by a Balancing Charger, such as the Turnigy Accucel 6. It charges the LiPo but measure all the cell voltages at the same time and can charge/discharge 1 cell individually if it falls behind/goes over the others.

You need not worry if you measure a Pack and find that the highest cell might be 4.21V when charged and the lowest 4.18V. 0.03V won't cause an issue. But I would be concerned if it was more like 4.2V and 4.0V.